Canon EOS 6D Instruction Manual page 107

i: Setting the ISO SpeedN
If [z4: Highlight tone priority] is set to [Enable], you cannot select "L"
(equivalent to ISO 50), ISO 100/125/160, "H1" (equivalent to ISO 51200),
and "H2" (equivalent to ISO 102400) (p.130).
Shooting in high temperatures may result in images that look grainier.
Long exposures can also cause irregular colors in the image.
When you shoot at high ISO speeds, noise (such as dots of light and
banding) may become noticeable.
When shooting in conditions that produce an extreme amount of noise,
such as a combination of high ISO speed, high temperature and long
exposure, images may not be recorded properly.
As H1 (equivalent to ISO 51200) and H2 (equivalent to 102400) are
expanded ISO speed settings, noise (dots of light, banding, etc.) and
irregular colors will be more noticeable, and the resolution lower than
usual.
If you use a high ISO speed and flash to shoot a close subject,
overexposure may result.
If you shoot a movie when [Maximum: 25600] is set with [ISO speed
range] and ISO speed is set to ISO 16000/20000/25600, the ISO speed
will switch to ISO 12800 (during movie shooting with manual exposure).
Even if you switch back to still photo shooting, the ISO speed will not
revert to the original setting.
If you shoot a movie when 1. L (ISO 50) or 2. H1 (ISO 51200)/H2 (ISO
102400) is set, the ISO speed setting will switch to 1. ISO 100 or 2. H
(ISO 25600) respectively (during movie shooting with manual exposure).
Even if you switch back to still photo shooting, the ISO speed will not
revert to the original setting.
Under [z3: ISO speed settings], you can use [ISO speed range] to
expand the settable ISO speed range from ISO 50 (L) to ISO 102400
(H2) (p.109).
<h> can be displayed in the viewfinder when you set an expanded ISO
speed (L, H1, or H2) (p.313).
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